1. Field of the Invention
This invention relates generally to a system and method for acquiring a distant terminal for transmitting an optical data beam thereto and, more particularly, to a system and method for mutually acquiring two distant terminals, where each terminal simultaneously scans an uncertainty region in which the other terminal is located, and where the scan beams include both encoded information about which section of the uncertainty region the scan beam is currently scanning and the position in the uncertainty region decoded from the other scan beam at the time the terminal of origin was last illuminated.
2. Discussion of the Related Art
Optical beams are sometimes employed to transmit digital data between two distant sources or terminals, such as space-to-space and space-to-ground optical communications, at very low power levels. The optical data beams have low divergence, and thus have extremely narrow beamwidths (for example 1-20 microradians) when they reach the target terminal to operate at the desired low power level. The optical data beam impinges on a collection aperture on the terminal. Conventional optics are used to focus the optical beam onto a detector to extract the data.
When the terminals wish to exchange data, and currently aren't tracking each other, an acquisition technique is employed to align the terminals so that the optical beam is transmitted in the proper direction with high accuracy. First, both terminals are informed of the general vicinity of the other terminal, defined herein as an uncertainty region. The uncertainty region is much larger than the angular beamwidth of the communications data beam. The terminals then initiate the acquisition process so that at the conclusion, their beam is pointed directly at the other terminal's telescope to exchange the data. During the acquisition process, information is exchanged between the two terminals until the uncertainty region is reduced to less than half the angular beamwidth of the data beam. If the incoming and outgoing beams are perfectly co-aligned, tracking can then commence to maintain the alignment. Because the information extracted by a local sensor from the arriving beam only tells the direction of the incoming beam, a misalignment of greater than one-half of the beamwidth between the incoming and outgoing beam will result in a failure to achieve track even if the incoming beam knowledge is perfect.
FIG. 1 is an illustration of a communications system 10 for transmitting optical data between a first terminal 12 and a second terminal 20 some distance apart. The terminal 12 includes a telescope having a sensor 14 for receiving optical beams from the terminal 20 and a transmitter 16 for transmitting optical beams to the terminal 20. Likewise, the terminal 20 includes a telescope having a sensor 22 for receiving optical beams from the terminal 12 and a transmitter 24 for transmitting optical beams to the terminal 12.
The terminal 12 gives its best estimate of its location to the terminal 20, and the terminal 20 gives its best estimate of its location to the terminal 12 for subsequent data transmissions. However, neither of the terminals 12 and 20 will give their location to the other terminal 12 or 20 with a high enough accuracy. Therefore, the actual location of the terminals 12 and 20 is unknown to the other terminal 12 or 20 before a signal beam is acquired. The given position of the terminals 12 and 20 is shown here as terminals 12′ and 20′, which is some unknown distance from the actual location of the terminals 12 and 20. Thus, an uncertainty region 28 is defined for the terminal 12 in which the terminal 12 is located, and an uncertainty region 30 is defined for the terminal 20 in which the terminal 20 is located. For example, the uncertainty region 28 or 30 may be 100 times the diameter of the data beamwidth, which provides a factor of 10,000 times of total area. The expected location of the terminals 12 and 20 is set at the center of the uncertainty regions 28 and 30, respectively, in two dimensions.
The uncertainty region 28 is shown relative to the sensor 22 of the terminal 20′, and the uncertainty region 30 is shown relative to the sensor 14 of the terminal 12′. The uncertainty defined by the uncertainty regions 28 and 30 includes both positional uncertainty and angular uncertainty. It is the angular uncertainty that causes the uncertainty regions 28 and 30 to be shown relative to the sensors 14 and 22 for the expected locations of the terminals 12′ and 20′.
In one known acquisition technique, the uncertainty regions 28 and 30 are flooded with a beacon of light from the terminals 12 and 20, respectively, to determine the position of the other terminal 12 or 20 by looking for the direction of the other terminals flood beam. This technique requires a separate beam than the data beam, and is typically relatively slow at providing acquisition if the power levels are low. Particularly, because of the distance between the terminals, the sensors 14 and 22 see the other terminals flood beam as a point source on its detector, such as a charge coupled device (CCD) array. It may take a significant amount of time for the CCD array receiving the low level flood beam to integrate enough charge to provide an indication of the direction of the other terminals flood beam.
Some of the problems with beacon type acquisition have been alleviated by employing scan beams that scan the uncertainty regions 28 and 30, where the beams are detected by the sensors 14 and 22, respectively. FIG. 2 shows the system 10 where only the actual locations of the terminals 12 and 20 are provided for illustrating a scan acquisition technique. The terminal 12 transmits a scan beam 34 from the transmitter 16, having the same beamwidth as the data beam, that is scanned across the uncertainty region 30 to illuminate the terminal 20. At the same time, the terminal 20 transmits a scan beam 36 from the transmitter 24 that scans across the uncertainty region 28 to illuminate the terminal 12. Each time the scan beam 34 or 36 is received by the sensor 22 or 24 of the terminal 12 or 20, that terminal 12 or 20 knows the approximate direction of the other terminal 12 or 20 because of where the beam impinges the sensors field-of-view. Thus, the terminals 12 and 20 can home in on each other by receiving the other terminals scan beam 34 or 36 until acquisition is completed. This occurs when the uncertainty regions 28 and 30 are reduced to less than half the beamwidth of the scan beams 34 and 36.
FIG. 3 depicts the terminals 12 and 20 after being acquired by the scan beams 34 and 36, after which the terminals 12 and 20 can track each other to maintain the pointing in the event that one or both of the terminals 12 or 20 is moving. Because the terminals 12 and 20 are simultaneously scanning for the other terminal 12 or 20, the average acquisition time can be reduced because once one terminal 12 or 20 is illuminated by the other terminal 12 or 20 its uncertainty area is reduced allowing it to acquire faster.
Recent improvements have been made in the known scan acquisition techniques to more quickly acquire the terminal of interest. Particularly, U.S. patent application Ser. No. 09/481,924 titled “Satellite Optical Communication Beam Acquisition Techniques,” filed Jan. 13, 2000, assigned to the Assignee of this application and herein incorporated by reference, discloses one improvement. In this scan acquisition technique, the position of one terminal is determined by subdividing a sensor of the other terminal into sensor quads, and then continually subdividing each sensor quad after the terminal receives the scan beam until acquisition.
The operation of this acquisition technique will be discussed herein with reference to the terminal 12, the sensor 14 and the scan beam 36 from the terminal 20. FIG. 4 shows a sensor 40, representing the sensor 14 that is applicable for this purpose. The '924 application used a sensor divided into sensor quads for two-dimensional acquisition. However, for illustration purposes, this technique can be shown in only one-dimension with the sensor 40. The sensor 40 is separated into a first sensor half 42 and a second sensor half 44 separated by a line 46. The sensor halves 42 and 44 only determine if the scan beam 36 arrives through the portion of the uncertainty region 28 being watched by that sensor half. Because there are only two cells, high performance materials and electronics can be used to maximize sensitivity and minimize noise at reasonable cost. A CCD with the same level of performance would be extremely expensive. One example of a sensor suitable for this purpose is an InGaAs cell, well known to those skilled in the art.
As the scan beam 36 scans the uncertainty region 28, the scan beam 36 will eventually impinge the sensor 14. The sensor 40 includes suitable circuitry to determine which sensor half 42 or 44 is illuminated by the scan beam 36 (referred to as a “hit”). The sensor 40 will then adjust its field-of-view so that the line 46 falls half-way through the portion of the uncertainty region 28 that was previously covered by the sensor half 42 or 44 that was “hit” by the scan beam 36. For example, if the field-of-view of the sensor 40 is 16°, and the scan beam 36 is detected by one of the sensor halves 42 or 44, the sensor 40 will then move the center of its field-of-view to bisect the portion of the original field-of-view covered by the particular sensor half 42 or 44 that detected the beam 36.
By positioning the sensor 40 at the field-of-view for the sensor half 42 or 44, the uncertainty region 28 is cut in half, and now this half is covered by both of the sensor halves 42 and 44 around the line 46. When the sensor 40 is illuminated by the scan beam 36 again, it will again hit one of the two sensor halves 42 or 44, and thus the field-of-view can be divided in half again. This process is continued until the scan beam 36 is simultaneously detected by both sensor halves 42 or 44 at the line 46. For a two-dimensional scan, a quad cell sensor would be employed in this manner.
Once the uncertainty region 28 is reduced to half of the beamwidth of the scan beam 36, the transmitter 16 of the terminal 12 should be aligned with the sensor 22 of the terminal 20. However, the process described above requires that the sensor 14 be accurately aligned with the transmitter 16 because the transmitter 16 transmits the scan beam 34 in the direction just determined by the sensor 14. Misalignment between the sensor 14 and the transmitter 16 will induce an undetectable bias in uncertainty region 28 which may be greater than one-half of the beamwidth. Therefore, it is necessary to precisely align the transmitted beam to the center of the sensors field-of-view using precise alignment devices or applying an extremely stable structure.